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United States Patent |
5,528,103
|
Spindt
,   et al.
|
June 18, 1996
|
Field emitter with focusing ridges situated to sides of gate
Abstract
A gated field-emission structure contains a emitter electrode (46), an
overlying electrically insulating layer (48, and one or more
electron-emissive elements (52) situated in one or more apertures
extending through the insulating layer. A patterned gate electrode (50)
through which each electron-emissive element is exposed overlies the
insulating layer. Focusing ridges (54) are situated on the insulating
layer on opposite sides of the gate electrode. The focusing ridges, which
normally extend to a considerably greater height than the gate electrode,
cause emitted electrons to converge into a narrow band.
Inventors:
|
Spindt; Christopher J. (Menlo Park, CA);
Corcoran; Patrick A. (Oakland, CA)
|
Assignee:
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Silicon Video Corporation (San Jose, CA)
|
Appl. No.:
|
188855 |
Filed:
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January 31, 1994 |
Current U.S. Class: |
313/497; 313/309; 313/310; 313/336; 313/351; 313/422; 313/496 |
Intern'l Class: |
H01J 031/12 |
Field of Search: |
313/309,310,336,351,422,495,496,497,452
345/74,75
|
References Cited
U.S. Patent Documents
4008412 | Feb., 1977 | Yuito et al.
| |
4020381 | Apr., 1977 | Oess et al.
| |
4178531 | Dec., 1979 | Alig.
| |
4618801 | Oct., 1986 | Kakino.
| |
4857799 | Aug., 1989 | Spindt et al. | 345/74.
|
4874981 | Oct., 1989 | Spindt.
| |
4884010 | Nov., 1989 | Biberian.
| |
4983878 | Jan., 1991 | Lee et al.
| |
5015912 | May., 1991 | Spindt et al.
| |
5070282 | Dec., 1991 | Epsztein.
| |
5155416 | Oct., 1992 | Suzuki et al.
| |
5164632 | Nov., 1992 | Yoshida et al.
| |
5227691 | Jul., 1993 | Murai et al.
| |
5235244 | Aug., 1993 | Spindt.
| |
5315207 | May., 1994 | Hoeberechts et al.
| |
Foreign Patent Documents |
0395158 | Oct., 1990 | EP.
| |
0523702 | Jan., 1993 | EP.
| |
0550335 | Jul., 1993 | EP.
| |
92/09095 | May., 1992 | WO.
| |
Other References
Spangenberg, Vacuum Tube, (McGraw-Hill), pp. 354-355, 1948.
|
Primary Examiner: Patel; Nimeshkumar D.
Attorney, Agent or Firm: Skjerven, Morrill, MacPherson, Franklin and Friel, MacPherson; Alan H., Meetin; Ronald J.
Claims
We claim:
1. A structure comprising:
an emitter electrode;
an electrically insulating layer situated over the emitter electrode;
a set of at least one electron-emissive element situated in at least one
aperture extending through the insulating layer down to the emitter
electrode such that each electron-emissive element contacts the emitter
electrode;
a gate electrode situated over the insulating layer, at least one opening
extending through the gate electrode to expose each electron-emissive
element; and
a pair of focusing ridges situated over the insulating layer on opposite
sides of, and spaced laterally apart from, the gate electrode, the
focusing ridges being sufficiently close to the gate electrode to control
trajectories of electrons emitted from each electron-emissive element, the
focusing ridges extending to an average height above the insulating layer
of at least ten times the average height of the gate electrode above the
insulating layer.
2. A structure as in claim 1 wherein the average height of the focusing
ridges above the insulating layer is at least one tenth of the spacing
between the focusing ridges.
3. A structure as in claim 1 further including an electrically conductive
section situated above, and spaced apart from, the gate electrode and
focusing ridges, the conductive section having an electron-receptive site
for receiving electrons emitted from each electron-emissive element.
4. A structure as in claim 3 wherein the electron-receptive site emits
light when struck by electrons from each electron-emissive element.
5. A structure as in claim 1 wherein the set of at least one
electron-emissive element comprises a multiplicity of electron-emissive
elements, each situated in a different aperture extending through the
insulating layer.
6. A structure as in claim 1 wherein each ridge comprises a metal bar.
7. A structure as in claim 6 wherein each ridge includes a highly resistive
electrically conductive coating over top and side surfaces of the metal
bar.
8. A structure as in claim 1 wherein each ridge comprises a dielectric bar.
9. A structure as in claim 8 wherein each ridge includes a metal film on
top of the dielectric bar.
10. A structure as in claim 8 wherein each ridge includes a metal coating
over top and side surfaces of the dielectric bar.
11. A structure as in claim 8 wherein each ridge includes a highly
resistive electrically conductive coating over top and side surfaces of
the dielectric bar.
12. A structure comprising:
an emitter electrode;
an electrically insulating layer situated over the emitter electrode;
an array of laterally separated sets of electron-emissive elements, each
set comprising at least one electron-emissive element situated in at least
one opening extending through the insulating layer down to the emitter
electrode such that each electron-emissive element contacts the emitter
electrode;
a plurality of electrically conductive gate lines extending over the
insulating layer largely in a primary direction, openings extending
through the gate lines to expose the electron-emissive elements; and
a plurality of focusing ridges extending over the insulating layer largely
in the primary direction, the focusing ridges being interdigitated with
the gate lines such that each gate line is largely situated between, and
laterally spaced apart from, a different consecutive pair of the focusing
ridges, the focusing ridges extending to an average height above the
insulating layer of at least ten times the average height of the gate
lines above the insulating layer.
13. A structure as in claim 12 wherein the average height of the focusing
ridges above the insulating layer is at least one tenth of the average
spacing between the focusing ridges.
14. A structure as in claim 12 further including:
an electrically conductive section situated above, and spaced apart from,
the gate lines and focusing ridges, the conductive section comprising an
array of electron-receptive sites respectively corresponding to the sets
of electron-emissive elements for receiving electrons emitted from the
electron-emissive elements; and
a support section that keeps the conductive section spaced apart from the
gate lines and focusing ridges.
15. A structure as in claim 14 wherein the electron-receptive sites emit
light when struck by electrons from the electron-emissive elements.
16. A structure as in claim 14 wherein the emitter electrode comprises a
plurality of emitter lines extending in a further direction substantially
different from the primary direction.
17. A structure as in claim 16 wherein the primary and further directions
are laterally orthogonal to one another.
18. A structure as in claim 12 wherein the ridges are electrically
conductive.
19. A structure as in claim 18 further including means for electrically
interconnecting the focusing ridges in order to apply substantially the
same voltage to all of them.
20. A structure as in claim 18 further including means for simultaneously
providing different voltages to different ones of the focusing ridges.
21. A structure as in claim 12 further including an additional plurality of
focusing ridges situated over the insulating layer, extending in a further
direction substantially different from the primary direction, meeting the
first-mentioned focusing ridges, and crossing over the gate lines.
22. A structure comprising:
an emitter electrode;
an electrically insulating layer situated over the emitter electrode;
an array of laterally separated sets of electron-emissive elements, each
set comprising at least one electron-emissive element situated in at least
one opening extending through the insulating layer down to the emitter
electrode such that each electron-emissive element contacts the emitter
electrode;
a plurality of electrically conductive gate lines extending over the
insulating layer largely in a primary direction, openings extending
through the gate lines to expose the electron-emissive elements;
a plurality of first focusing ridges extending over the insulating layer
largely in the primary direction, the first focusing ridges being
interdigitated with the gate lines such that each gate line is largely
situated between, and laterally spaced apart from, a different consecutive
pair of the first focusing ridges, the first focusing ridges extending to
an average height above the insulating layer of at least ten times the
average height of the gate lines above the insulating layer; and
a plurality of second focusing ridges extending over the insulating layer
in a further direction substantially different from the primary direction,
meeting the first focusing ridges and crossing over the gate lines.
23. A structure as in claim 22 further including;
an electrically conductive section situated above, and spaced apart from,
the gate lines and focusing ridges, the conductive section comprising an
array of electron-receptive sites respectively corresponding to the sets
of electron-emissive elements for receiving electrons emitted from the
electron-emissive elements; and
a support section that keeps the conductive section spaced apart from the
gate lines and focusing ridges.
24. A structure as in claim 23 wherein the electron-receptive sites emit
light when struck by electrons from the electron-emissive elements.
25. A structure as in claim 23 wherein the emitter electrode comprises a
plurality of emitter lines extending in the further direction.
26. A structure as in claim 25 wherein the primary and further directions
are laterally orthogonal to one another.
27. A structure as in claim 22 wherein the ridges are electrically
conductive.
28. A structure comprising:
an emitter electrode;
an electrically insulating layer situated over the emitter electrode;
a set of at least one electron-emissive element situated in at least one
aperture extending through the insulating layer down to the emitter
electrode such that each electron-emissive element contacts the emitter
electrode;
a gate electrode situated over the insulating layer, at least one opening
extending through the gate electrode to expose each electron-emissive
element;
a pair of first focusing ridges situated over the insulating layer on
opposite sides of, and spaced laterally apart from, the gate electrode,
the first focusing ridges being sufficiently close to the gate electrode
to control trajectories of electrons emitted from each electron-emissive
element, the first focusing ridges extending to an average height above
the insulating layer of at least ten times the average height of the gate
electrode above the insulating layer; and
a pair of second focusing ridges situated over the insulating layer,
meeting the first focusing ridges, and crossing over the gate electrode.
29. A structure as in claim 28 further including an electrically conductive
section situated above, and spaced apart from, the gate electrode and
focusing ridges, the conductive section having an electron-receptive site
for receiving electrons emitted from each electron-emissive element.
30. A structure as in claim 29 wherein the electron-receptive site emits
light when struck by electrons from each electron-emissive element.
31. A structure as in claim 28 wherein the set of at least one
electron-emissive element comprises a multiplicity of electron-emissive
elements, each situated in a different aperture extending through the
insulating layer.
32. A structure as in claim 28 wherein each ridge comprises a metal bar.
33. A structure as in claim 32 wherein each ridge includes a highly
resistive electrically conductive coating over top and side surfaces of
the metal bar.
34. A structure as in claim 28 wherein each ridge comprises a dielectric
bar.
35. A structure as in claim 34 wherein each ridge includes a metal film on
top of the dielectric bar.
36. A structure as in claim 34 wherein each ridge includes a metal coating
over top and side surfaces of the dielectric bar.
37. A structure as in claim 34 wherein each ridge includes a highly
resistive electrically conductive coating over top and side surfaces of
the dielectric bar.
Description
FIELD OF USE
This invention relates to electron-emitting devices. More particularly,
this invention relates to gated field-emission devices suitable for
products such as cathode-ray tube ("CRT") displays of the flat-panel type.
BACKGROUND ART
A gated field-emission device (or field emitter) is an electronic device
that emits electrons when subjected to an electric field of sufficient
strength. The electrons are extracted from an electron-emissive element by
a gate electrode, and are subsequently collected at an anode spaced apart
from the electron-emissive element and gate electrode. An area field
emitter contains a group, often a very large group, of individual
electron-emissive elements distributed across a supporting structure. Area
field emitters are employed in CRTs of flat-panel televisions.
Referring to the drawings, FIG. 1 generally illustrates part of a
conventional flat-panel CRT containing a field-emission backplate (or
baseplate) structure 10 and an electron-receiving faceplate structure 12.
Backplate structure 10 commonly consists of an electrically insulating
backplate 14, an emitter (or base) electrode 16, an electrically
insulating layer 18, a patterned gate electrode 20, and a conical
electron-emissive element 22 situated in an aperture through insulating
layer 18. The tip of electron-emissive element 22 is exposed through a
corresponding opening in gate electrode 20. Emitter electrode 16 and
electron-emissive element 22 together constitute a cathode for the
illustrated part of the CRT. Faceplate structure 12 is formed with an
electrically insulating faceplate 24, an anode 26, and a coating of
phosphors 28.
Anode 26 is maintained at a positive voltage relative to cathode 16/22. The
anode voltage is typically 300-700 volts for a conventional spacing of
100-200 .mu.m between structures 10 and 12. Because anode 26 is in contact
with phosphors 28, the anode voltage is impressed on phosphors 28. When a
suitable gate voltage is applied to gate electrode 20, electrons are
emitted from electron-emissive element 22 at various values of off-normal
emission angle .theta.. The emitted electrons follow parabolic
trajectories indicated by lines 30 in FIG. 1 and impact on a target
portion 28T of phosphors 28. The phosphors struck by the emitted electrons
produce light of a selected color.
Phosphors 28 are part of a picture element ("pixel") that contains other
phosphors (not shown) which emit light of different color than that
produced by phosphors 28. Also, the pixel containing phosphors 28 adjoins
one or more other pixels (not shown) in the CRT. If some of the electrons
intended for phosphors 28 consistently strike other phosphors (in the same
or another pixel), the image resolution and color purity are degraded.
The size of target phosphor portion 28T depends on the applied voltages and
the geometric/dimensional characteristics of the CRT. Although the
anode/phosphor voltage is typically 300-700 volts in the conventional
flat-panel display of FIG. 1, power efficiency and phosphor lifetime are
both considerably higher at a phosphor potential of 1,500-10,000 volts.
However, increasing the anode/phosphor voltage to 1,500-10,000 volts in
the CRT of FIG. 1 would require that the spacing between backplate
structure 10 and faceplate structure 12 be much greater than the
conventional value of 100-200 .mu.m. Increasing the inter-structure
spacing to the value needed for a phosphor potential of 1,500-10,000 volts
would, in turn, cause target phosphor portion 28T to become too large for
a commercially viable flat-panel CRT display.
Focusing electrodes have been placed above the gate electrodes in field
emitters to improve image resolution. For example, see U.S. Pat. Nos.
4,178,531, 5,070,282, and 5,235,244. Unfortunately, relatively complex
processing at micrometer or submicrometer scale dimensions is usually
needed to create a focusing electrode above the gate. It would be
desirable to have a relatively simple gated field-emission structure that
achieves high image resolution and color purity at high anode/phosphor
voltage.
GENERAL DISCLOSURE OF THE INVENTION
The present invention furnishes a gated field-emission structure that
utilizes focusing ridges situated to the sides of the gate for causing
emitted electrons to converge into a narrow band. In flat-panel CRT
applications of the present field-emission structure, high image
resolution and color purity are achievable at a phosphor potential of
1,500-10,000 volts where power efficiency and phosphor lifetime are high.
The focusing ridges can be fabricated in a straight-forward manner without
complex processing at micrometer or submicrometer scale dimensions.
Accordingly, the invention provides a substantial advance over the prior
art.
Specifically, the field-emission structure of the invention contains an
emitter electrode, an overlying electrically insulating layer, and a set
of one or more electron-emissive elements situated in one or more
apertures extending through the insulating layer down to the emitter
electrode. A gate electrode is situated over the insulating layer. One or
more openings extend through the gate electrode to expose each
electron-emissive element.
A pair of focusing ridges are situated over the insulating layer on
opposite sides of the gate electrode. The focusing ridges are spaced
laterally apart from the gate electrode. However, the ridges are close
enough to the gate electrode to influence the trajectories of electrons
emitted from each electron-emissive element. The ridges normally extend to
a greater height than the gate electrode. The potentials of the ridges are
controlled in such a way that a high percentage of the electron
trajectories bend into a small band. Consequently, the image resolution
and color purity are high when the field-emission structure is employed in
a flat-panel CRT.
The invention is readily extended to an area field emitter. In doing so,
the gate electrode becomes a plurality of gate lines extending over the
insulating layer in one direction. Electron-emissive elements are situated
in apertures through the insulating layer and are exposed through openings
in the gate lines. A plurality of focusing ridges extend over the
insulating layer in the same direction as the gate lines. The focusing
ridges are interdigitated with the gate lines such that each gate line is
situated between, and laterally spaced apart from, a pair of the focusing
ridges. The emitter electrode becomes a plurality of emitter lines
extending in a different direction than the gate lines and focusing
ridges.
With proper design, the focusing ridges can handle electrons emitted at
large off-normal angles. Large energy spread due to current-limiting
resistors can also be handled by the ridges without significant loss in
image resolution or color purity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-section structural view of part of a prior art flat-panel
CRT display that utilizes a gated field emitter.
FIG. 2 is a cross-sectional structural view of part of a flat-panel CRT
display that utilizes a gated field emitter having focusing ridges in
accordance with the invention.
FIG. 3 is a plan view of the part of the backplate structure in the CRT of
FIG. 2. The cross section of FIG. 2 is taken through plane 2--2 in FIG. 3.
FIG. 4 is a plan view representing the full extent of the backplate
structure in the CRT of FIG. 2.
FIG. 5 is a cross-sectional structural view representing the full extent of
the backplate and faceplate structures in the CRT of FIG. 2. The cross
section of FIG. 5 is taken through plane 5--5 in FIG. 4.
FIG. 6 is a plan view representing a full-width part of the faceplate
structure in the CRT of FIG. 2. Plane 5--5 in FIG. 6 likewise indicates
the cross section through which FIG. 5 is taken.
FIG. 7 is a plan view of part of an alternative backplate structure for a
flat-panel CRT that utilizes focusing ridges in accordance with the
invention.
FIGS. 8.1, 8.2, 8.3, 8.4, 8.5, and 8.6 are cross-sectional structural views
of focusing ridges employable in the CRTs of FIGS. 2 and 7.
FIG. 9 is a plan view of part of an alternative backplate structure for a
flat-panel CRT that employs crossing groups of focusing ridges in
accordance with the invention.
Like reference symbols are employed in the drawings and in the description
of the preferred embodiments to represent the same or very similar item or
items.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 2 generally illustrates part of a flat-panel CRT that employs focusing
ridges to improve image resolution and color purity in accordance with the
invention. The CRT in FIG. 2 contains a field-emission backplate (or
baseplate) structure 40 and an electron-receiving light-emissive faceplate
structure 42. The interior surfaces of structures 40 and 42 face each
other and are typically 0.1-2.5 mm apart. FIG. 3 depicts a top view of the
portion of backplate structure 40 shown in FIG. 2.
The illustrated part of backplate structure 40 is formed with an
electrically insulating backplate 44, a metallic emitter (or base)
electrode 46, an electrically insulating layer 48, a metallic gate
electrode 50, a multiplicity of electron-emissive elements 52, and a pair
of focusing ridges 54. Backplate 44 is a flat plate typically consisting
of glass, ceramic, or silicon. Emitter electrode 46 lies on the upper (or
interior) surface of backplate 44 and is typically formed with molybdenum
or chromium. Emitter electrode 46 is in the shape of a line whose width
w.sub.E is typically 100 .mu.m. Insulating layer 48 lies on emitter
electrode 46 and on the laterally adjacent portion of backplate 44. Layer
48 typically consists of silicon dioxide. Components 44-48 typically have
respective thicknesses of 1.0 mm, 0.5 .mu.m, and 1.0 .mu.m.
Gate electrode 50 lies on insulating layer 48. As indicated in FIG. 3,
electrode 50 is in the shape of a line running perpendicular to emitter
electrode 46. The width w.sub.G of gate electrode 50 is preferably 30
.mu.m. Electrode typically 50 has an average height (or thickness) h.sub.G
of 0.02-0.2 .mu.m. Electrode 50 typically consists of a
titanium-molybdenum composite.
Electron-emissive elements 52 extend through apertures in insulating layer
48 to contact emitter electrode 46. The tips (or upper ends) of
electron-emissive elements 52 are exposed through corresponding openings
56 in gate electrode 50. Electron-emissive elements 52 can have various
shapes. Although elements 52 are illustrated as needle-like elements in
FIG. 2, they could (for example) be cones. The shape of elements 52 is not
particularly material here as long as they have good electron-emissive
characteristics.
Electron-emissive elements 52 are distributed across part or all of the
portion of gate electrode 50 overlying emitter electrode 46. FIG. 3
illustrates the case in which elements 52 occupy a portion 50A of
electrode 50 situated above electrode 46. The width w.sub.A of active
emitter-area gate portion 50A in FIG. 3 is less than the width w.sub.G of
electrode 50, while the length l.sub.A of active area portion 50A largely
equals the width w.sub.E of emitter electrode 46. Also, active-area width
w.sub.A in FIG. 3 is approximately centered on gate width w.sub.G. Item b
in FIG. 3 indicates the border spacing between one of the edges of
electrode 50 and the corresponding longitudinal edge of portion 50A. The
areal density of elements 52 is typically 1 element/.mu.m.sup.2. Elements
52 in combination with emitter electrode 46 form part of the cathode for
the CRT.
Electron-emissive elements 52 can be manufactured according to various
processes, including those described in Macaulay et al, U.S. patent
application Ser. No. 08/118,490, filed 8 Sep. 1993, now U.S. Pat. No.
5,462,467 and Spindt et al, U.S. patent application Ser. No. 08/158,102,
filed 24 Nov. 1993, now allowed. The contents of Ser. Nos. 08/118,490 and
08/158,102 are incorporated by reference herein.
Depending on how elements 52 are fabricated, openings 58 may extend through
gate electrode 50 at locations where insulating layer 48 lies directly on
backplate 44. Because openings 58 do not overlie emitter electrode 46, no
electron-emissive elements are exposed through openings 58. If present,
openings 58 therefore do not significantly affect device operation.
Focusing ridges 54 lie on insulating layer 48. As shown in FIG. 3, focusing
ridges 54 are in the shape of bars situated on the opposite sides of, and
running in the same direction as, gate electrode 50. Accordingly, ridges
54 also extend perpendicular to emitter electrode 46.
The width w.sub.F of each ridge 54 is approximately 25 .mu.m. Ridges 54 are
spaced equidistantly apart from gate electrode 50. The electrode-to-ridge
spacing s.sub.L preferably is 25 .mu.m. The total spacing s.sub.F between
ridges 54 equals w.sub.G +2s.sub.L and thus preferably is 80 .mu.m.
Focusing ridges 54 normally extend to a considerably greater height above
insulating layer 48 than gate electrode 50. Preferably, the average height
h.sub.F of ridges 54 is at least ten times the average height h.sub.G of
gate electrode 48. More preferably, h.sub.F is at least 100 times h.sub.G.
The ratio h.sub.F /s.sub.F of ridge height to ridge spacing preferably is
at least 0.1 and, more preferably, is at least 0.4. Typically, h.sub.F is
20-50 .mu.m.
The illustrated part of faceplate structure 42 is formed with an
electrically insulating faceplate 60, a pair of dark non-reflective lines
62, a patterned coating of phosphors 64, and a thin light-reflective layer
66. Faceplate 60 is a flat plate typically consisting of glass.
Dark lines 62 are situated on the lower (or interior) surface of faceplate
60 respectively opposite focusing ridges 54. Lines 62 are black or nearly
black and, when struck by electrons, are substantially non-emissive of
light relative to phosphors 64. The width w.sub.M of lines 62 is usually
approximately the same as the width w.sub.F of ridges 54.
Phosphors 64 lie on the remaining portions of the lower surface of
faceplate 60. A target portion 64T of phosphors 64 is situated between
dark lines 62 opposite gate electrode 50. Target phosphor portion 64 has a
width w.sub.T approximately equal to s.sub.F. Portions 64S of phosphors 64
are situated on the other sides of dark lines 62.
Light-reflective layer 66 lies on phosphors 64 and dark lines 62 along
their lower (or interior) surfaces. The thickness of layer 66 is
sufficiently small, typically 50-100 nm, that nearly all electrons from
electron-emissive elements 52 pass through layer 66 with little energy
loss. Part of the light emitted by phosphors 64 is reflected by layer 66
through faceplate 60. Also, layer 66 consists of a metal, preferably
aluminum, and thereby acts as the anode for the CRT.
Depending on the design of the CRT, focusing ridges 54 can be maintained at
one voltage or at different voltages. Typically, the voltage on each ridge
54 is close to the voltage of emitter electrode 46. Light-reflective layer
66 and, consequently, phosphors 64 are maintained at a voltage of
1,500-10,000 volts, preferably 4,000-10,000 volts, relative to the
emitter-electrode voltage. When electron-emissive elements 52 are
activated, the gate voltage is typically 10-40 volts higher than the
emitter voltage.
Electron-emissive elements 52 emit electrons at off-normal emission angle
.theta. when gate electrode 50 is provided with a suitably positive
voltage relative to the emitter-electrode voltage. The emitted electrons
move towards phosphors 64 (and dark lines 62) along trajectories indicated
by lines 68. When struck by these electrons, phosphors 64 emit light of
selected color.
Focusing ridges 54 influence trajectories 68 in such a way that target
phosphor portion 64T is struck by substantially all emitted electrons for
which emission angle .theta. is less than or equal to a specified maximum
value .theta..sub.MAX. Typically, .theta..sub.MAX is
40.degree.-60.degree.. This provides increased image resolution and color
purity at a phosphor voltage of 1,500-10,000 volts because the width
w.sub.T of target portion 64T can be made smaller than the width of
electron-target areas in otherwise similar conventional flat-panel CRTs.
Setting ridge height h.sub.F at a value much greater than gate height
h.sub.G provides several benefits. The large negative focus voltage
(typically several hundred volts) needed when h.sub.F equals h.sub.G is
greatly reduced. The width w.sub.A of gate emitter area 50A can be
increased, thereby enabling the areal density of electron-emissive
elements 52 to be increased. Also, internal supports (not shown) are
typically placed between backplate structure 40 and faceplate structure 42
to maintain a constant inter-structure spacing across the CRT. By making
h.sub.F much greater than h.sub.G, ridges 52 can provide contact sites
along backplate structure 40 for the internal supports and thus avoid
having the internal supports contact, and possibly damage, critical thin
films such as gate electrode 50.
In the full implementation of the CRT of the invention, backplate structure
40 contains an array of emitter-electrode lines 46, gate-electrode lines
50, and focusing ridges 54. Turning to FIG. 4, it illustrates the
characteristics of the full layout of the array formed by emitter lines
46, gate lines 50, and ridges 54 in structure 40. Gate lines 50 and ridges
54 are interdigitated with one another and run in a direction
perpendicular to emitter lines 46. Gate lines 50 extend through the wall
at one end of the array, while ridges 54 extend through the wall at the
opposite end of the array.
Focusing ridges 54 are connected to focus control circuitry 70 as
schematically shown in FIG. 4. Focus control circuitry 70 controls the
potentials on ridges 54 in one of two general ways depending on CRT
design.
One of the control techniques is to place focusing ridges 54 at the same
voltage by connecting them all together. In this case, circuitry 70 simply
controls the value of the single ridge voltage.
The other control technique is to divide ridges 54 into a number of
equal-size consecutive groups. The first (e.g., left-most) electrodes in
these groups of ridges 54 are connected together to receive one voltage
whose value can vary. The second electrodes in the ridge groups are
connected together to receive another variable voltage. When the group
size is three or more, the third electrodes are connected together to
receive a third variable voltage, and so on. Circuitry 70 then operates as
a multiplexer for controlling the values of the ridge voltages in response
to suitable control signals. This control technique is discussed further
below in connection with FIGS. 5 and 6.
FIG. 5 depicts a full cross section of structures 40 and 42 when backplate
structure 40 is laid out as shown in FIG. 4. As indicated in FIG. 5, an
outer wall 72 is situated between structures 40 and 42 outside the active
picture area. Outer wall 72 supports structures 40 and 42 and helps keep
them separated from each other. The full CRT structure typically also
includes the above-mentioned internal supports (again not shown) which
ensure that the spacing between structures 40 and 42 is uniform across the
entire active area of the CRT. The interior CRT pressure is typically
below 10.sup.-7 torr.
Structures 40 and 42 are subdivided into an array of rows and columns of
pixels. The boundaries of a typical pixel 74 are indicated by dotted lines
in FIG. 4 and by corresponding boundary markers in FIG. 5. Each emitter
line 46 is a row electrode for one of the rows of pixels. Each column of
pixels has three of gate lines 50: (a) one for red (R), (b) a second for
green (G), and (c) the third for blue (B). Each pixel column utilizes four
of focusing ridges 54. Two of ridges 54 are internal to the pixel column.
One or both of the remaining two are shared with the pixel(s) in the
adjoining column(s).
FIG. 6 illustrates the characteristics of a full-width portion of the
layout of faceplate structure 42 in the CRT of FIG. 2. Structure 42
contains a group of dark lines 62 and a group of stripes of phosphor 64
arranged in an alternating pattern. Dark lines 62 constitute a "black
matrix". As indicated by typical pixel 74 in FIG. 6, each column of pixels
contains a stripe of phosphors 64 that emit red light, a stripe of
phosphors 64 that emit green light, and a stripe of phosphors 64 that emit
blue light.
Pixel 74 has a width w.sub.P and a length l.sub.P normally equal to
w.sub.P. From an examination of FIGS. 2-6, w.sub.P equals 3(w.sub.M
+w.sub.T) which, in turn, equals 3(w.sub.F +s.sub.F). Preferably, w.sub.P
and l.sub.P are both 315-320 .mu.m.
Focusing ridges 54 in the full implementation of FIGS. 4-6 improve the
image resolution and color purity in the row direction (i.e., along the
rows of pixels) in the manner discussed above in connection with FIGS. 2
and 3. The image resolution is less critical in the column direction
(i.e., along the columns of pixels) because the length l.sub.T of the
phosphor target 64T, while being somewhat greater than the length l.sub.A
of active area portion 50A of each gate line 50, is considerably less than
the length l.sub.P of each pixel. Preferably, l.sub.T is approximately 200
.mu.m. Consequently, l.sub.T is more than 100 .mu.m less than l.sub.P.
Also, the color purity is not a problem in the column direction because
the color is the same in going along each phosphor stripe 64 in a pixel
column.
When the second of the above-mentioned control techniques (i.e., the one in
which focus control circuitry 70 functions as a multiplexer) is utilized
in the full CRT of FIGS. 4-6, focusing ridges 54 situated directly to the
left of "red" gate lines 50 receive one ridge voltage. Ridges 54 located
directly to the left of "green" gate lines 50 receive another ridge
voltage. Finally, ridges 54 situated directly to the left of "blue" gate
lines 50 receive a third ridge voltage.
Focus control circuitry 70 controls the values of the three ridge voltages
in such a way that electrons from field emitters 52 extending through gate
lines 50 for one of the three colors are directed toward corresponding
target phosphors 64T of that color. Electrons from emitters 52 extending
through gate lines 50 for the other two colors are simultaneously
collected on ridges 54 situated directly between those lines 50. By so
utilizing ridges 54 to perform both an electron-focusing function and an
electron-collecting function, only electrons intended to cause phosphors
64 to emit light of one color are provided from emitters 52 at a time. To
achieve all three colors, the CRT is operated frame sequentially.
Focusing ridges 54 can be configured to improve image resolution in the
column direction. Turning to FIG. 7, it depicts an alternative layout of a
portion of backplate structure 40 containing a full pixel 74. In this
alternative, ridges 54 have widened portions 54W situated between emitter
lines 46. Widened portions 54W cause electrons emitted from
electron-emissive elements 52 to converge closer to the vertical centers
of phosphor targets 64T. FIG. 7 also shows that elements 52 can be located
in portions 50A of gate lines 50 where (a) the width w.sub.A of each
portion 50A is less than the width w.sub.G of gate lines 50 and/or (b) the
length l.sub.A of each portion 50A is less than the width w.sub.E of
emitter lines 46.
Focusing ridges 54 can be formed with a number of different types of
materials ranging from electrical insulators to metals, and can be
configured in a variety of ways. FIGS. 8.1-8.6 depict typical structures
for ridges 54.
In FIG. 8.1, each focusing ridge 54 consists of a metal bar 54M. In FIG.
8.2, each ridge 54 is formed with metal bar 54M and a highly resistive
electrically conductive coating 54RC.
FIG. 8.3 illustrates an example in which each focusing ridge 54 consists of
a dielectric bar 54D. In FIG. 8.4, each ridge 54 is formed with dielectric
bar 54D and resistive coating 54RC. In FIG. 8.5, each ridge 54 consists of
dielectric bar 54D and a metal film 54MF on top of dielectric bar 54D. In
FIG. 8.6, each ridge 54 is formed with dielectric bar 54D and a metal
coating 54MC.
In manufacturing the CRT of the invention, components 44-52 in backplate
structure 40 can be fabricated in a conventional manner. Components 44-52
can, as indicated above, also be made according to the techniques
described in U.S. patent applications Ser. Nos. 08/118,490 and 08/158,102,
cited above.
In an embodiment where focusing ridges 54 utilize metal bars such as in
FIGS. 8.1 and 8.2, thin bottom portions of the metal bars can be created
from the same metal as gate lines 50 by depositing a layer of appropriate
metal on insulating layer 48 and then patterning the metal using a
suitable photoresist mask to simultaneously create gate lines 50 and the
bottom portions of the metal bars. The remainders of the metal bars can be
electroplated on the bottom portions using a photoresist mask to cover
gate lines 50. Alternatively, the remainders of the metal bars can be
created by placing a suitable pre-patterned metal screen over the bottom
portions of the metal bars. The screen wires that form the remainders of
the metal bars can be square or circular in cross section.
Components 60-64 in backplate structure 42 can be fabricated in a
conventional manner. Alternatively, components 60-64 can be manufactured
in accordance with the techniques described in Curtin et al, commonly
owned U.S. patent application Ser. No. 08/188,856, filed 31 Jan. 1994
contents of which are incorporated by reference herein.
The CRT preferably contains the above-mentioned internal supports (not
shown) for supporting the CRT against atmospheric pressure and maintaining
a uniform spacing between structures 40 and 42. The internal supports can
be fabricated in a conventional manner, in accordance with Fahlen et al,
commonly owned U.S. patent application Ser. No. 08/012,542, filed 1 Feb.
1993, or in accordance with Fahlen et al, commonly owned U.S. patent
application Ser. No. 08/188,857 filed 31 Jan. 1994 "Structure and The
contents of these two patent applications are incorporated by reference
herein. Outer wall 72 is provided to complete the basic CRT fabrication.
While the invention has been described with reference to particular
embodiments, this description is solely for the purpose of illustration
and is not to be construed as limiting the scope of the invention claimed
below. For example, gate lines 50 could be extended through the walls at
both ends of the array by providing suitable cross-over connections for
focusing ridges 54. Pre-formed screen wires that implement ridges 54 could
have cross sections other than square or circular.
An anode that directly adjoins faceplate 60 could be utilized in place of,
or in conjunction with, light-reflective layer 66. Typically, such an
anode would be used when the anode/phosphor voltage is 1,500-4,000 volts.
Elements other than phosphors 64 could be utilized as electron-receptive
light-emissive sites in faceplate structure 42. Instead of being flat,
backplate 44 and faceplate 60 could be curved.
Each gate line 50 could be employed with three (consecutive) phosphor
stripes 64. The CRT could then be operated using focusing ridges 54 to
deflect and focus electrons onto each of the three target portions 64
under the control of focus control circuitry 70.
If additional focusing is needed in the column direction beyond the extra
column-direction focusing provided in the alternative layout of FIG. 7,
widened portions 54W of adjacent ridges 54 could be connected together to
form focusing ridges extending in the row direction. In that case, the
focusing ridges extending in the row direction would cross over emitter
lines 50 and would be separated from them by an additional dielectric
layer. FIG. 9 illustrates such an embodiment of the invention using the
topography of FIG. 7 except that widened portions 54W are replaced with
additional focusing ridges 76 that extend perpendicularly to, and meet,
focusing ridges 54. Various modifications and applications may thus be
made by those skilled in the art without departing from the true scope and
spirit of the invention as defined in the appended claims.
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